Bar screens have proven particularly valuable in sorting materials which have unequal dimensions. Wire or punched screens are typically used to sort materials of a granular nature in which all three dimensions are approximately equal. However, many classes of objects, including two of particular commercial interest, wood chips and municipal or industrial trash, are not readily amenable to separation by conventional screening processes.
In the manufacture of paper, logs are reduced to wood chips by chipping mechanisms, and the chips are cooked with chemicals at elevated pressures and temperatures to remove lignin. The chipping mechanisms produce chips which vary considerably in size and shape. For the cooking process, which is known as digesting, it is desirable that the chips supplied have a uniform thickness in order to achieve optimal yield and quality. Ideally, the supplied chips will allow production of a pulp which contains a low percentage of undigested and/or over-treated fibers. Thus, a means is needed to separate chips on the basis of thickness rather than any other dimension. Bar screens have proven particularly adept at separating materials based on a single dimension such as thickness.
With the rise in the recycling culture, a strong demand for an apparatus for separating municipal and industrial trash into its constituent components for recycling has developed. Conventional separation systems which utilize rotating screen drums have proved ineffective. Municipal trash typically contains a certain portion of stranded material and sheet-like materials which tend to clog screens. Further, the tumbling action of screens can result in the breakage of components of the municipal waste stream such as glass bottles thereby increasing the difficulty of recycling them.
Bar screens consist of two sets of generally rectangular bars which are joined together in an array of racks. The two sets of bars are interleaved to form a screening bed. The bed consists of the elongated, rectangular bars and the narrow, rectangular spaces between the bars. Material to be sorted is introduced to the surface of the bed and the bars are caused to oscillate so that when one set of bars is going up, the other set is going down. This oscillatory motion tends to tip wood chips or other relatively small planar objects on edge so that those of a given thickness may slide through the gaps between the bars. Alternatively, it has been found when separating office waste paper, that bar screens prove effective in removing extraneous litter from the recovered office paper.
If the limitations of current bar screens could be overcome, the utility of the bar screen, already a valuable tool in the pulp industry and in the recycling industry, would be greatly increased. The first limitation relates to capacity. It is always desirable in a screening apparatus to increase the rate at which materials may be fed over the screen and yet be properly processed by the screen. In the case of bar screens, the existing capability of a given screen is dependent on the total area of the screening bed and more particularly the area of the gaps between the bars through which the separated material must pass. Thus, it would be advantageous to increase both the size of a bar screening unit and the total open area between bars. In current bar screens sets of bars are mounted on shafts which are driven eccentrically. Eccentric shafts, however, can only be of a limited length before the bending loads on the shafts cause excessive bearing wear. Further, the narrow screening bars tie together structurally the eccentric shafts. Hence increasing the screen open area by reducing the width of the bars is impractical because of the resultant reduction in structural stiffness of the bars.
Other areas of possible improvement in bar screens are associated with the desirability of maintaining strict timing between the eccentric drives of each set of bars so that they are maintained at a consistent 180 degrees out of phase relation.
Lastly, reduced maintenance and improved ease of maintenance are always desirable in industrial machinery, particularly those which must function in a dirty environment.
What is needed is a bar screen of increased capacity, improved timing linkages, and lower maintenance costs.